Magnetic microspheres and cenospheres recovered from fuel ashes were studied by SEM, XRD, electron probe microanalysis, chemical analysis and Mossbauer spectroscopy. The magnetic microspheres were shown to consist of a crystal phase based on a solid solution of Fe3O4-Mg, Mn, Ca-ferrite, alpha-Fe2O3 and a glass phase composed of calcium ferrite, fayalite, wollastonite, ferrous olivines and mullite. The morphology of globules and size of crystallites of active phase were established to depend on the basicity of the glass phase. It was shown that catalytic activity of magnetic microspheres and cenospheres in oxidative conversion of methane is determined by component containing iron and depends on its accessibility and distribution on the surface.